Fix/3860 nested formatter precision

[way4sahil]

Fixes #3860

Dynamic width and precision specifications inside nested_formatter now
resolve argument IDs against the enclosing format context.

This adds:

• Context forwarding for nested formatters.
• Dynamic width handling.
• Named and explicit argument support.
• Tests for dynamic width, precision, padding, and static formatting.
• A ChangeLog entry.

[fallenmi]

Requesting changes at exact head ee0b93c72d2ecd933d24997304bf6e6de2488a0a.

The dynamic-precision path itself works when the formatter uses the new nested(value, ctx) overload: my exact-base oracle (21d4cc15c8a2a3536aab5623258ba3b3dae573ce) reports argument not found, while this head produces (1.00, 2.00). The focused head test also passes (format-test --gtest_filter=format_test.nested_formatter, 1/1, 9.5 s including configure/build).

There are two attributable blockers on this head:

1. The repository's g++-13 c++23 Release Shared job fails in format.h:4178: the new local alias using context = ... shadows the existing fmt::context, and the project promotes -Wshadow to an error. Please rename that alias (for example, nested_context_type).
2. The clang-format job fails around the same specialization. I reproduced this with Apple clang-format 21 and the repository config: the exact base produces no diff, while the exact head rewrites lines 4171-4181. Please apply clang-format 21 to the touched code.

Please also drop unrelated commit d0d9adaf6ee72213ad2f9925b7056d5eb2890467 from this branch. Its C++20 module documentation change is already being reviewed separately in #4891 and is unrelated to the nested_formatter fix.

Disclosure: this review was prepared with Codex assistance; I verified the diff, exact SHAs, test outputs, and current CI logs before submitting it.

[vitaut]

Please remove unrelated commits and fix lint.

[way4sahil]

Requesting changes at exact head ee0b93c72d2ecd933d24997304bf6e6de2488a0a.

The dynamic-precision path itself works when the formatter uses the new nested(value, ctx) overload: my exact-base oracle (21d4cc15c8a2a3536aab5623258ba3b3dae573ce) reports argument not found, while this head produces (1.00, 2.00). The focused head test also passes (format-test --gtest_filter=format_test.nested_formatter, 1/1, 9.5 s including configure/build).

There are two attributable blockers on this head:

1. The repository's `g++-13 c++23 Release Shared` job fails in `format.h:4178`: the new local alias `using context = ...` shadows the existing `fmt::context`, and the project promotes `-Wshadow` to an error. Please rename that alias (for example, `nested_context_type`).

2. The `clang-format` job fails around the same specialization. I reproduced this with Apple clang-format 21 and the repository config: the exact base produces no diff, while the exact head rewrites lines 4171-4181. Please apply clang-format 21 to the touched code.

Please also drop unrelated commit d0d9adaf6ee72213ad2f9925b7056d5eb2890467 from this branch. Its C++20 module documentation change is already being reviewed separately in #4891 and is unrelated to the nested_formatter fix.

Disclosure: this review was prepared with Codex assistance; I verified the diff, exact SHAs, test outputs, and current CI logs before submitting it.

Thank you for the review. Fixed and committed. Please re-review @fallenmi @vitaut

[fallenmi]

Re-reviewed exact head 1fbe5d5a310819071d745f196ddfffe3d0fd4cc2 against current main b039e624c02ae473280156235217f9e02792b055 and synthetic merge 7b1eb1d338d717d46316ced2bb739dfe77a29f62.

My previous blockers are resolved: the shadowing alias is now nested_context_type, both g++-13 c++23 Release Shared and clang-format pass, and the branch rewrite removed the unrelated module-documentation commit. The context forwarding and dynamic width/precision handling are coherent, with coverage for automatic, positional, and named arguments plus output-iterator formatting.

The exact-head rollup is fully green: 47/47 checks across 8 suites and 7 workflows. I found no new blocker.

Disclosure: this review was prepared with Codex assistance; I verified the current refs, merge diff, review history, source changes, and CI through public GitHub data.

[vitaut]

Closing in favor of cbb2641 but thanks for the PR.